1. Field of the Invention
The present invention relates to a semiconductor device including a field effect transistor.
2. Description of the Related Art
In order to achieve high functionality of integrated circuits, elements forming the integrated circuits, that is, MISFETs (Metal-Insulator-Semiconductor Field Effect Transistors) and CMISFETs (Complementary-MISFETs) need to have high performance. The MISFET is a metal-insulator-semiconductor field effect transistor and the CMISFET is a complementary metal-insulator-semiconductor field effect transistor. Basically, high performance of these elements has been achieved according to scaling law. However, various problems have recently arisen due to the limitations of material properties. To describe one of the foregoing problems, there is a problem that it is difficult to control the threshold voltage of a MISFET.
Under the present circumstances, the threshold voltage is controlled by the impurity concentration of channel regions. However, if the foregoing method is employed, carrier mobility is reduced; for this reason, the on/off characteristic of the MISFET is degraded. A fully-depleted MISFET is given as the method of achieving high performance of the MISFET except for scale-down. However, the threshold voltage control for the MISFET is not achieved according to the method of controlling the impurity concentration in channel only.
In view of the foregoing reason, threshold voltage is currently attempted to control by using the work function of gate electrode materials. In the coming technical generation, as well as in the current one, different work functions are required between gate electrodes of n-type and p-type MISFETs, and it is needed that the polycrystalline silicon gate electrodes be replaced with metal electrodes to lower the resistance of the gate electrodes. However, if quite different gate electrode materials are used, the manufacturing method becomes complicated, and the cost increases. For this reason, the following method of controlling the work function has been proposed. According to the method, the same kind of compounds having different composition and concentration are used as the gate electrodes of n-type and p-type MISFETs, and thereby, the work function is controlled.
For example, various methods given below have been reported. One is a method of controlling the work function using the composition ratio of Ru and Ta of an RuTa alloy (see Jaehoon Lee et al., IEDM Tech. Dig., 2002 p. 359-362). Another is a method of using the composition ratio of NiSi and CoSi of (NiCo)Si (see J. Kedzierski et al., IEDM Tech. Dig., 2002 p. 247-250). Another is a method of using each impurity concentration of B, P and As contained in NiSi (see the same as above). Another is a method of using the composition ratio of Si and Ge of SiGe (see JPN. PAT. APPLN. KOKAI Publication No. 2002-43566).
The gate electrode material is required to have a stable correlation between the composition of the material or variations of dopant concentration and the work function. The condition is satisfied, and thereby, the control of the work function is improved with respect to the required threshold voltage. In devices including both n-type and p-type MISFETs on the same substrate, the variable range of the work function preferably includes each range required for these n-type and p-type MISFETs. In this way, it is possible to prevent complication of the manufacturing method and an increase of the cost. However, the foregoing methods do not meet the requirements described above.
More specifically, the correlation is given between the metal composition of the RuTa alloy and the work function. However, heat resistance is low, and the work function irregularly varies by heat treatment of about 500° C. or more at least. Thus, the correlation is thermally unstable. In addition, the heat treatment is carried out, and thereby, Ru or Ta of the gate electrode diffuses into the channel region. As a result, there is a problem that the on/off characteristic of the MISFET is degraded.
The compound (NiCo)Si has no stable correlation between the composition ratio of Ni:Co and the work function. The variable range of the work function is narrow, that is, about 0.2 eV.
The compound NiSi has a correlation between each impurity concentration of B, P and As contained therein and the work function. The variable range of the work function is sufficient with respect to fully-depleted MISFET devices. However, the variable range of the work function is insufficient to obtain a sufficiently low threshold voltage in a bulk MISFET device, which is not the fully-depleted MISFET device.
If SiGe is used as the gate electrode material, a correlation is given between the composition ratio of Si and Ge and the work function. However, the variable range of the work function is about 4.7 eV to about 5.2 eV. As a result, the compound SiGe is applied to a p-type MISFET only in the current technical generation.
Therefore, it is greatly desired to realize a device having a stable correlation between variation of gate electrode material and work function, in the devices having both n-type and p-type MISFETs on the same substrate. In other words, it is desired in the device to realize a semiconductor device including a gate electrode, which has a variable range of a work function including a range required for both n-type and p-type MISFETs.